I checked several web sites and a number of states allow towing at 75mph on Interstates.  A number don't even reduce the speed for 18 wheelers like Illinois does.  Years ago, we couldn't keep Carlisle tires on boat trailers and the switch to speed rated LOAD RANGE B radials solved the problem. Next time you see a full size van, see if it even has truck tires on it.  You might be surprised to see how many don't , but just have LR "B" car tires.  Same with trucks.  A new Ford F-150 Crew Cab is available with 17" car tires that only carry a maximum of 35 pounds of air for a smooth ride. Some where on this board are notes from people who are using truck tires of the proper load range and size.